Mylar (a trademark of E. I. DuPont deNemours) is a well known material, and will be referred to hereafter as PET. It is a polyester which has ester groups only in the backbone of the hydrocarbon chain comprising its polymer. Commercially, it is a very useful material that is used as a medium for audio and video tapes, and for many aspects of semiconductor circuitry, including circuit boards. It is characterized by high strength and excellent chemical stability.
Although PET is a very useful material, it is difficult to provide intricate patterns in it, because a chemical which dissolves it in a controlled way does not exist. Patterning of PET is desirable for many reasons, such as the fabrication of circuits and to prevent PET films from clinging excessively to tape heads in tape applications. Previously, some patterning of PET was achieved using a long wavelength laser to modify the surface of the PET films by intense local heating. Such a technique is described in U.S. Pat. Nos. 3,549,733; 3,617,702; 3,920,951; and in German Pat. No. 2,115,548. In all of these patents, wavelengths in excess of about 250 nm were used. The mechanism to modify the PET films depended upon localized thermal effects from the heat of the laser beam.
It is also desirable to modify the surface of PET in order to make it more wettable. This promotes adhesion of PET to other films, such as metal films or other polymer films. In order to modify the surface of the PET films to promote wettability, an electric discharge has been passed across the surface of the film while a reactive inorganic vapor was present, as described in U.S. Pat. No. 3,255,099. The electric discharge had no effect unless the vapor was also present.
Photochemical decomposition of PET films involves the use of laser beams to cause photochemical reactions in the PET film. The following references are examples of this:
1. M. Day et al., J. Appl. Polymer Sci. 16, (1972), a series of three articles I-III, beginning on pages 175, 191, and 203, and a fourth article IV by M. Day et al. appearing in this journal, Vol. 17, pp. 1895-1907 (1973). PA1 2. F. B. Marcotte et al., J. Polymer Sci. 55, 477 (1961). PA1 3. F. B. Marcotte et al., J. Polymer Sci. Part A-1, Vol. 5, 481-501 (1967).
The Marcotte et al. references describe the photolysis of PET with ultraviolet radiation having a wavelength greater than 250 nm. The purpose of that laboratory investigation was to study the weathering of this polymer. As noted from those studies, this ultraviolet irradiation produced degradation of the polymer, the most effective wavelength causing degradation being 3130 A. During photolysis, the gaseous products formed are CO and CO.sub.2. Additionally, the polymer molecules are split and cross-linked and different radicals are formed. On page 498 of Reference 3, the authors note that the quantum yields for CO and CO.sub.2 formation are similar for light having wavelengths 2537 and 3130 A. Additionally, it is stated that the gross chemical effects are similar for these different wavelengths, although they occur in a much thinner layer for 2537 A light than for 3130 A light.
The effect of long wavelength radiation in the weakening of the PET polymer is described by D. J. Carlsson and D. M. Wiles in the text "Ultraviolet Light Induced Reactions in Polymers," Ed. S. S. Labana, ACS Symposium, Serial No. 25, page 321 (1976).
The series of four articles I-IV listed in Reference 1 (M. Day et al.) describes an extensive laboratory analysis of the photochemical degradation of PET during ultraviolet irradiation having wavelengths 220-420 nm. In these references, it is noted that molecular chain breaking and degradation occcur with ultraviolet radiation of any wavelength, and that the longer wavelengths are absorbed throughout the bulk of the polymer, while the shorter wavelengths are absorbed much more readily by the polymer. The main volatile products are CO and CO.sub.2, and appear to be produced more readily when the photolysis occurs in the presence of oxygen.
Article IV of this series describes the surface changes which occurred during the ultraviolet irradiation of the PET polymer. On page 1902 of this article, it states that the textural and physical damages at the surface of the polymer were minimal, even after prolonged uv exposure, although some volatilization appeared to take place from the irradiated regions (page 1904). However, progressive deterioration of the bulk of the polymer occurred, as noted on page 1906.
Thus, although Articles I-IV describe in detail many of the types of chemical reactions which occur during uv irradiation of PET, no drastic surface changes were noted. As noted in these references, all wavelengths of ultraviolet radiation cause chain-breaking and degradation of the polymer. However, as applicants have discovered, the character of the photochemistry occurring during ultraviolet irradiation changes remarkably when the wavelengths of radiation are less than 220 nm (although Article II lists the wavelength range as 200-400 nm, this is a loose designation, the lowest wavelength used being 220 nm as determined by the filter used in the optical system of that reference). There appears to be a threshold at a wavelength of less than 220 nm where the molecular chain-breaking process becomes much more efficient and in which the bulk of the polymer does not become degraded. At wavelengths less than 220 nm, surface changes rapidly occur and uv irradiation at wavelengths less than 220 nm causes photoetching of the surface of the PET. This photoetching occurs rapidly and can be used to produce patterns in the PET, and to completely remove PET from the irradiated regions in a short amount of time. It is this much more greatly efficient photochemical process that underlies applicants' invention.
U.S. Pat. No. 4,247,496 describes a method for treating a thin surface layer of a plastic material, such as PET. In this patent the plastic material is subject to an ultraviolet light treatment, after which it is stretched. The ultraviolet light has a wavelength ranging from 180 to 400 nm, and is emitted by sources such as mercury lamps, fluorescent lamps, xenon lamps, and carbon-arc lamps.
In the patent described in the previous paragraph, the phenomonon which occurs is one which is described in the aforementioned Reference IV to Blais, Day and Wiles. The ultraviolet light treatment causes cracking in a surface layer (50-100 .ANG.) of the plastic. These cracks make stretching easier, and leave a surface which contains widened cracks therein.
In U.S. Pat. No. 4,247,496 no photoetching is involved, because it is important that only a thin surface layer be affected. U.S. Pat. No. 4,247,496 does not recognize that selected wavelengths of ultraviolet light can be used to efficiently photoetch PET. In that patent, a broad wavelength range of ultraviolet light is used to affect only a thin surface layer of the PET in order to enhance stretching of the PET.
In contrast with this, the invention of the present application is directed to photoetching of PET by a range of ultraviolet radiation having wavelengths less than 220 nm. What is desired is effective photoetching of PET and not merely a surface treatment. In this invention, specified far uv is used to etch PET without affecting the bulk of the material and without causing undue heating. In fact, several of the lamps suitable in the practice of the invention described in U.S. Pat. No. 4,247,496 cannot be used in the present invention. These unsuitable uv sources include high pressure mercury lamps, xenon lamps, and carbon-arc lamps. In further contrast with that patent, the present invention relies on the use of far uv radiation to photoetch through a substantial portion of the PET, wherein low power can be used to effectively photoetch. For example, 1,000 .ANG. of the PET can be removed per ArF excimer laser pulse in 12 nsec., where the pulse has a power of 150 mJ.
It is recognized that any uv irradiation of a plastic material will cause degradation of the material after a long time. For example, the reference to Blais et al describes degradation effects which occur over very prolonged exposure (for example, thousands of hours). In contrast with this, the present invention is directed to efficient photoetching of PET where the active wavelengths for this process must be &lt;220 nm.
It would be advantageous to have a suitable process for photoetching polyesters such as PET without adversely weathering the material. Accordingly, it is a primary object of the present invention to provide a technique for the photochemical decomposition of polyesters including PET to a depth at least about 1,000 .ANG. to bring about changes in its properties, without adverse weathering effects.
It is another object of the present invention to provide a technique for photoetching the surface of polyesters including PET without the use of localized heating.
It is another object of the present invention to provide a technique for photochemical decomposition of PET, without degrading and weathering the PET.
It is another object of the present invention to provide a self developing technique for photoetching polyesters, such as PET.
It is a further object of this invention to provide a technique for selective photoetching the surface of PET without the requirement for any liquid solvents.
It is another object of the present invention to provide a technique for rapid etching of polyesters without relying on thermal effects.
It is a still further object of this invention to provide a photoetching treatment for photochemically decomposing a layer of PET without modifying the bulk of the material.
It is a still further object of this invention to provide a photochemical surface treatment of PET which has reaction rates many orders of magnitude greater than prior photochemical techniques.
It is another object of this invention to provide a self developing photoetching technique that can be used to provide precise patterns of any depth in polyesters, such as PET.